Call extension report for Computer and tablet
Call-extensions run on all devices so you cannot set a phone number to appear with your search ad only for mobile searches. Those clicks are most likely on the headline of the Ad or on other extensions.
To get more precise data use the Segment button and select "This extension vs. other" and you will see exactly how many clicks the call extension received. When I select this segment I see 0 clicks on the "call extension" and all the clicks are attributed to "other".
In addition, I would like to add that you can segment the data of Call Extensions by click type. Doing this would give you more specific data as to which clicks went to the headlines, which to your other ad extensions like sitelinks, Get Direction, Mobile click to call etc...
My 2 cents!